Bathroom & Kitchen Exhaust Fan CFM Calculator

How it works

Bath: CFM = volume × 8 ÷ 60, or 1 CFM/ft² (≤ 100 ft²), or 50 CFM per fixture (> 100 ft²), minimum 50; kitchen: 100 CFM per linear ft of wall hood (150 island) or gas BTU/h ÷ 100, minimum 100; equivalent duct length = length + 15 ft per elbow

Bathroom fans are sized to change the room air roughly eight times an hour, which for ordinary bathrooms works out to about one CFM per square foot with a 50 CFM floor set by ASHRAE 62.2 and the residential codes; larger bathrooms are sized per fixture because it is the shower and tub that make the steam. Range hoods scale with the width of the hood and the heat of the burners — gas cooking releases combustion products as well as grease and moisture, so one CFM per hundred BTU/h is the rule, and island hoods need half again more because cross-drafts pull the plume away. Both fans lose much of their rated flow to duct friction, so the duct size and length matter as much as the box, and hoods above 400 CFM trigger a make-up-air requirement so the kitchen does not depressurise the house and backdraft the furnace.

Worked example

A 10 × 8 ft bathroom with 8 ft ceilings needs 85 CFM by the 8 ACH rule (80 by the 1 CFM/ft² rule), so a fan rated about 110 CFM through 45 equivalent feet of 4–6″ duct. A 30″ wall hood over a 45,000 BTU/h gas range needs 450 CFM by the BTU rule (250 by width) — above 400, so make-up air is required.

Frequently asked questions